The invention relates to a process for the treatment of a cracking gas produced in the cracking of 1,2-dichloroethane to give vinyl chloride.
In processes for the preparation of vinyl chloride by incomplete cracking of 1,2-dichloroethane (EDC), the EDC employed is usually evaporated in the first step, then, in a second step, the vapor formed is cracked pyrolytically at relatively high temperature, furthermore, in a third step, the entrained solids are separated off from the hot cracking gas produced in the second step, and subsequently the purified cracking gas is fed to distillative treatment.
The main products formed in the EDC cracking carried out in the second process step are hydrogen chloride (HCl) and vinyl chloride (VCM):
C2H4Cl2+heatxe2x86x92HCl+C2H3Cl
By-products formed in traces are soot, chlorinated and unsaturated hydrocarbons and benzene. In order to limit the formation of these undesired by-products, the cracking temperature is held at a level which results in incomplete reaction of the EDC. The hot cracking gas produced by cracking in the second process step therefore also contains unreacted 1,2-dichloroethane (EDC) in addition to the main products hydrogen chloride (HCl) and vinyl chloride (VCM) and said by-products.
The cracking of EDC to give VCM is an endothermic process. It takes place in the gas phase in the form of pyrolysis. In industry, the pyrolysis is carried out without a catalyst under high pressure of from 1 to 3 MPa and at a temperature of from 450 to 600xc2x0 C. However, work is also being carried out on catalytic processes which allow the pyrolysis to be carried out at lower pressure and lower temperature. The hot cracking gas produced by means of pyrolysis is formed at the pyrolysis temperature. It has to be conditioned so that it takes on a form which is suitable for the actual substance separation.
Before the actual substance separation of the cracking gas, the cracking gas heat is therefore utilized economically in one or more heat exchangers. In the process, the temperature of the cracking gas in the case of catalyst-free pyrolysis drops from between 480 and 540xc2x0 C. to between about 180 and 280xc2x0 C.
In a company publication published by Uhde GmbH in June 1995 with the title xe2x80x9cVinyl chloride plants/Hoechst processxe2x80x9d, the process usual hitherto for the treatment of the cracking gas is described.
In the process flow chart reproduced on page 11 of this company publication, it is shown that the precooled cracking gas is then cooled further and partially condensed in a quench. To this end, a cooled cracking gas condensate is introduced at the top of the quench zone. Two take-off streams containing product (VCM) are formed in the quench zone from the cracking gas and the cracking gas condensate introduced at the top:
the bottom discharge product is formed from the discharge product flowing out at the base of the quench zone;
the vapors flowing out at the top of the quench zone form the quench gas.
Only partial condensation of the cracking gas takes place in the quench zone. The liquid running out of the quench zone at the bottom, the so-called bottom discharge product, therefore contains as principal constituents 1,2-dichloroethane, vinyl chloride and high-boiling components and the solids, i.e. soot and/or coke.
The quench gas flowing out at the top of the quench zone as vapors contains hydrogen chloride, vinyl chloride and 1,2-dichloroethane as principal constituents. It is more or less free from the solids, i.e. free from soot and/or coke.
The cracking gas condensate employed as quench liquid is usually branched off from the condensed quench gas as a sub-stream, causing the formation of a quench liquid circuit.
The bottom discharge product and the partially condensed quench gas are then treated further.
EP-0 276 775-B1 gives the temperature of the catalyst-free pyrolysis as from 450 to 550xc2x0 C. and pyrolysis pressures of from 0.5 to 3 MPa, but preferably from 1.6 to 2.6 MPa, and gentle measures for substance stream guidance.
DE-23 13 037-C3 describes a gentle arrangement of the evaporation.
Since it has proven advantageous to separate off the solids particles entrained by the cracking gas therefrom before distillative treatment of the cracking gas, this is usually carried out together with its cooling and partial condensation in the quench zone. In general, a quench device of simple design, which essentially consists of a vertical tank and a device for atomization of quench liquid in the interior of the tank, generally effects virtually complete purification of the cracking gas. The solids removed accumulate at the bottom of the quench zone.
U.S. Pat. No. 5,558,746 describes a quench column with plates which has a complex design and in which the solids are likewise removed from the circuit together with the bottom discharge product and then separated off. At the same time, the condensation of the cracking gas, which is usually carried out in an external device, is integrated into the quench column.
As is known, the pyrolysis of 1,2-dichloroethane is highly endothermic and characterized by the consumption of large amounts of thermal energy.
There has therefore been no lack of proposals for recovery of as much as possible of the heat present in the hot cracking gas from pyrolysis by means of heat transfer to other media:
EP-0 276 775-B1 indicates four variants for utilization of the cracking gas heat content for combinations of prewarming, evaporation and superheating of the EDC feed into the pyrolysis and for the generation of steam.
DE-31 47 310-C2 indicates the utilization of the heat content of the cracking gas for steam generation or for the heating of bottom forced-circulation evaporators in the distillative treatment in the preparation of VCM.
EP-0 180 995-B2 uses the product stream freed from solids in a quench column for generating steam and warming the EDC feed for the pyrolysis.
A common feature of the above-mentioned process variants is that the pyrolytically generated hot cracking gas is either cooled substantially to the vicinity of the dew point or even condensed at the dew point and therefore has to be re-heated in the subsequent distillative treatment. Even in the case of comprehensive use of the above-described heat recovery measures, there remains on the one hand a significant remainder of unutilized heat lost to the environment due to cooling and on the other hand a deficit of heat which has to be supplied again if necessary.
In the above-mentioned company publication with the title xe2x80x9cVinyl chloride plants/Hoechst processxe2x80x9d published by Uhde GmbH in June 1995, the process usual hitherto for the distillative treatment of the cracking gas is described. In the process flow chart reproduced on page 13 of this company publication under the heading xe2x80x9cVCM distillationxe2x80x9d, it is shown how the cracking gas taken off from the quench zone and freed from solids is then treated to give VCM, the distillative treatment of the pyrolytically generated cracking gas principally being directed toward the actual substance separation of this three-substance system with the three principal components HCl, VCM and unreacted EDC.
Three-substance systems require a comparatively complex separation apparatus for their separation. In addition, the present three-substance system also has a very broad boiling range, which means that the separation task requires solutions with a greater energy requirement than a mixture having a narrower boiling range.
Between the boiling points of HCl (minus 85xc2x0 C.) and EDC (plus 83.5xc2x0 C.), there is a temperature difference of 168.5xc2x0 C. Owing to the very low boiling point of hydrogen chloride (HCl), cooling, preferably with cooling media which supply cold, must be carried out for the separation of HCl. In industry, the separation of HCl is carried out, for example, at 1.3 MPa absolute and xe2x88x9224xc2x0 C. The industrial generation of cooling media which release cold is significantly more complex and also more complex than the provision of a cooling medium which has to achieve heat dissipation from the temperature level of the environment.
In the previous processes for distillative treatment, as described in German patent specifications DE-12 50 426, DE-19 10 854-C3 and DE-43 42 042-A1, the hydrogen chloride (HCl) is removed at the top of the first distillation column. In order to restrict the demand for cold to an economically sensible level, it is vital to feed the pyrolytically generated cracking gases into the first distillation column in substantially condensed form.
Furthermore, the bottom discharge product taken off from the first distillation column in the previous process shown in the above-mentioned company publication from Uhde GmbH essentially contains the VCM regarded as product and the unreacted EDC. This bottom discharge product is fed to the second column. In the second column, it is separated into unreacted EDC and VCM. However, the VCM distilled off at the top of the second column contains unavoidable fractions of HCl, which have to be separated off in a third column using additional supply of energy.
The cause of the presence of HCl in the vapors taken off at the top of the second distillation column in the previous process is due, in particular, to the thermal instability of the EDC. By means of suitable, sufficiently sensitive analytical methods, the decomposition products HCl and VCM can be detected in pure EDC even at temperatures of from 100xc2x0 C. With increasing temperature and increasing residence time, the amount of HCl and VCM formed also increases.
The previous process therefore has considerable potential for further improvements. Firstly, the previous distillation sequence with removal of HCl in the first step does not allow substantial utilization of the heat present in the pyrolytically generated cracking gas, since the cracking gas must be substantially condensed in order to minimize the demand for cold energy.
Secondly, the removal of the VCM from all high-boiling components in the second step requires considerable expenditure of energy, together with the necessity, owing to the thermal instability of the EDC, of removing newly formed HCl in an additional purification step.
This is where the process according to the invention starts and has the object of improving the existing process by separating HCl and VCM off from all higher-boiling components in the first step of the distillation sequence and only carrying out the unavoidable use of coolant in the subsequent step for the removal of the HCl.
It thus becomes possible in accordance with the invention to avoid, in particular, the energy losses which usually occur due to substantial condensation of the quench gas and its reheating for the purpose of distillative separation and, through circulating the unreacted EDC at a higher temperature level, subsequently to increase the original conversion of the pyrolytic cracking of EDC.
To this end, the invention proposes a process for the treatment of the cracking gas, in which the cracking gas employed has been formed from the pyrolysis of 1,2-dichloroethane (EDC), the cracking gas is split into its principal components hydrogen chloride (HCl), vinyl chloride (VCM) and unreacted 1,2-dichloroethane (EDC), and these principal components arise in substantially pure form, where, in the first step of the treatment of the cracking gas, the solids are separated off therefrom in a quench zone, and the process is characterized in that:
the further treatment steps are carried out by means of a rectifying zone, a distillation zone and a stripping zone
and a solids-free cracking gas condensate which has an increased concentration of 1,2-dichloroethane (EDC) is used as quench liquid in the quench zone.
Embodiments of the invention arise from the further claims.
In the preferred embodiment, the invention relates to a process for the treatment of the cracking gas that arises in the non-catalytic thermal cracking of 1,2-dichloroethane to give vinyl chloride with a temperature of from 480 to 540xc2x0 C. and a pressure of from 0.5 MPa to 3 MPa and is optionally cooled to a temperature of from 180 to 280xc2x0 C. in a heat recovery and is passed with this temperature into a quench zone, in which it is cooled and washed with EDC-enriched cracking gas condensate, where the process is characterized in that:
1 to 2% by weight, based on the amount of cracking gas employed, are removed in liquid form as solids-loaded bottom discharge product at the bottom of the quench zone,
the remaining amount (cracking gas used plus quench liquid minus bottom discharge product) is removed in gas form as purified quench gas at the top of the quench zone,
the quench gas obtained here is passed directly downwards into a rectifying zone, and the quench gas is separated therein into a distillate and a bottom product, which is free from components which boil higher than VCM, and an EDC-enriched bottom product,
the bottom product arising in the rectifying zone is discharged, and the said solids-free cracking gas condensate, which is enriched with 1,2-dichloroethane (EDC), is thus obtained, and it is separated into two sub-streams, and, as the first sub-stream, the preliminary fraction for the stripping zone is obtained and, as the other sub-stream, the EDC-enriched, solids-free cracking gas condensate that is fed back into the quench zone in order to treat the cracking gas to be treated with it therein as coolant and washing agent is obtained;
and in that the distillate and the preliminary fraction are fed separately to the further distillative treatments
and the bottom discharge product coming from the quench zone which is loaded with the solid to be removed is fed to another treatment.
In the process according to the invention, the distillate intended for the first further distillative treatment and the preliminary fraction intended for the other further distillative treatment are obtained free from any solids loading. The significant advantage inherent in the previous process therefore also arises in the process according to the invention, namely that it is advantageous if it can be avoided that the solids emanating from the cracking gas, such as soot and/or coke, are able to enter the apparatuses of the further distillative treatments and deposit therein. The formation of blockages in practical operation of the apparatuses is countered in a simple manner of this type.
This is because, as already described above, the solids removed in the purification are, in the previous process, formed exclusively at the bottom of the quench zone, which enables simple concentration and disposal of the solids suspension.
It has proven particularly surprising in the process according to the invention that the quench gas entering the rectifying zone has, at an EDC cracking rate of at least 50% in the prior pyrolysis, such a composition that it is sufficient, at an absolute pressure of at least 1.9 MPa above the uppermost plate of the rectifying zone, to charge the reflux condenser of the rectifying zone merely with cooling water running at the ambient temperature in order to obtain a vigorous reflux into the rectifying zone. Furthermore, it has proven surprising in the process according to the invention that, with at least 25 theoretical plates for the rectifying zone and a reflux ratio of 1.5 to 2.5, firstly top vapor, reflux and distillate streams which are free from all components which boil higher than VCM, such as, for example, chloroprene or EDC, are obtained, and that secondly, the discharge product from the bottom plate of the rectifying zone is enriched with EDC to greater than 80% by weight since the EDC is that principal component of the three-substance mixture which has the highest boiling point.
It arises in accordance with the invention that the thermal instability of the EDC in a sub-stream which forms the preliminary fraction in the stripping zone are unharmful in respect of the purity requirements of the product VCM. If, as a consequence of the thermal instability, HCl and VCM form as decomposition products in the stripping zone, they are formed at the top of the stripping zone and, in accordance with the invention, are fed back from there into the rectifying zone. Gentle performance of the distillation, as was appropriate in the second step of the distillation in the previous process, is no longer necessary in accordance with the invention. In particular, the pressure and temperature at the bottom of the stripping zone can be increased without harm. In the rectifying zone, the decomposition products are again removed at the top and passed over into the distillation zone together. However, thermal instability cannot take effect in the distillation zone since the distillation zone is operated well below 100xc2x0 C. in all of its separation stages.
One embodiment of the process according to the invention therefore proposes that the distillative treatment of the preliminary fraction takes place in the stripping zone, the 1,2-dichloroethane (EDC) recovered in enriched form is taken off with the bottom product formed therein, and the top product formed therein is fed back into the rectifying zone.
Another embodiment of the process according to the invention therefore proposes that the distillative treatment of the distillate is carried out in the distillation zone, the vinyl chloride (VCM) separated off is taken off in highly enriched form with the bottom product formed therein, and the hydrogen chloride (HCl) is recovered in enriched form with the top product formed therein. For reasons of safe operation, in particular in order to intercept HCl breakthrough into the VCM product in the case of non-optimum operation of the column, a separate fine purification zone as in the previous process is appropriate.
The process according to the invention utilizes the heat content of the pyrolytically generated cracking gas virtually completely, in a particularly advantageous manner, through the quench gas being fed directly downwards into the rectifying column as vapor to be treated distillatively. Furthermore, the connection according to the invention of the quench zone to the newly arranged distillation sequence of the treatment results in circulation of the unreacted EDC at longer residence time at higher temperatures, for which reason, in combination with the thermal instability of the EDC, an unexpected increase in yield occurs without the need for an additional purification step.
An advantageous embodiment of the process can consist of the following steps:
firstly, the gas mixture to be separated can be present with temperatures of from 85 to 540xc2x0 C. and with a pressure of from 0.1 MPa to 3.0 MPa,
secondly, the gas mixture to be separated is, before entry into the first distillation column, fed in gas form beneath the internals of a rectifying zone,
thirdly, a mixture of HCl, VCM and components which boil lower than VCM is taken off at the top of the rectifying zone and passed as feed to a distillation zone,
fourthly, the bottom product from the rectifying zone is a mixture which consists of EDC as principal component and HCl, VCM and components which boil higher than VCM as secondary components and which is fed to a stripping zone as a sub-stream, if quench liquid is necessary for prior separation of solids from the gas mixture employed, or as a full stream,
fifthly, the top product from the rectifying zone is separated in the distillation zone into high-purity VCM, which is formed as bottom product in the distillation zone, and substantially pure HCl, which consists of HCl and components which boil lower than VCM and is formed at the top of the distillation zone, and
sixthly, the mixture running in from the bottom of the rectifying zone is separated in the stripping zone, substantially pure EDC being taken off at the bottom of the stripping zone together with all components boiling higher than VCM and with an only very low content of HCl and VCM, and a mixture consisting of EDC and HCl and VCM introduced into the feed and HCl and VCM newly formed due to the thermal instability of EDC being obtained at the top of the stripping zone and being fed back into the rectifying zone for the purposes of further treatment.
The process is also distinguished by the fact that a gas mixture employed in the rectifying zone, freed from solids, but provided with relatively low pressure, is brought, using a compression unit, to a pressure which ensures economical operation of the rectifying zone.
It may be advantageous for the stripping zone to be operated either at lower or at higher pressure than the pressure used in the rectifying zone. In the case of a lower pressure in the stripping zone, the pressure increase of the top product of the stripping zone which is fed back to the rectifying zone can either be carried out by total condensation of this top product with subsequent pressure increase via a pump or in the gaseous state by means of a compression unit. In the case of a higher pressure in the stripping zone, the pressure matching is carried out by means of a pump in the stripping zone feed originating from the bottom of the rectifying zone.
The process may also be distinguished by one or more bottom forced-circulation evaporators of the distillation zone being heated by means of the bottom product from the stripping zone, where it may likewise be provided that the bottom product from the distillation zone is fed through an optionally connectable fine purification zone, a high-purity product stream being taken off at the bottom of the fine purification zone, and the top product from the fine purification zone being fed back into the distillation zone.
The process may also be distinguished by the use of a gas mixture charged with solid particles, such as, for example, soot, as is usually produced in the non-catalytic thermal cracking of 1,2-dichloromethane to give vinyl chloride at a temperature of from 480 to 540xc2x0 C. and a pressure of from 0.5 MPa to 3.0 MPa and is optionally cooled to a temperature of from 180 to 280xc2x0 C. in a heat recovery step. In the case of catalytic cracking of 1,2-dichloroethane to give vinyl chloride, the gas mixture employed may also be present at lower temperatures of up to 85xc2x0 C. and at a lower pressure of up to 0.1 MPa.
In the case of solids charging of the gas mixture employed, the process is characterized in that a quench zone for washing out the solids particles and quench bottom product treatment for the recovery of EDC, VCM and HCl are provided before the separation devices (rectifying zone, distillation zone and stripping zone) described in claim 1.
The quench zone is characterized in that a sub-stream of the bottom discharge product from the rectifying zone is employed as quench liquid after cooling.
The quench zone is furthermore characterized in that
firstly, from 1 to 20% by weight, based on the amount of gas mixture employed, are produced in liquid form at the bottom of the quench zone as solids-loaded bottom discharge product and are treated further in the quench bottom product treatment,
secondly, the remaining amount (gas mixture employed plus quench liquid minus bottom discharge product) is produced at the top of the quench zone in gas form as purified quench gas and is fed into the rectifying zone either directly or via a compression unit according to claim 2,
thirdly, the quench zone can be defined in equipment terms either as an independent apparatus or alternatively in the form of a component integrated into the rectifying zone, and
fourthly, the quench zone is designed in equipment terms as an independent apparatus if the compression unit according to claim 6 is employed.
The quench bottom product treatment is characterized in that the solids-loaded bottom discharge product is thickened by stepwise decompression and evaporation to the extent that the residual stream obtained remains pumpable, substance streams which contain primarily EDC and also HCl and VCM being obtained during the evaporation and being fed into the stripping zone in order to recover its useful substances.
The quench bottom product treatment is furthermore characterized in that
firstly, in the first step, decompression is carried out to a pressure which is somewhat above the pressure in the stripping zone,
secondly, a gas stream is obtained by supplying heat to the first step and is fed directly into the stripping zone,
thirdly, the remaining liquid from the first step is decompressed to virtually atmospheric pressure in the second step, and
fourthly, a gas stream is obtained by supplying heat to the second step and, after total condensation, is brought to the pressure of the stripping zone by means of a pump and fed into this zone.
It is also advantageous that the gas mixture employed in the quench zone is provided with a lower pressure than the pressure prevailing in the stripping zone, and that, instead of the decompression in the first step of the quench bottom product treatment, the quench bottom product is brought to a somewhat higher pressure than the pressure prevailing in the stripping zone by means of a pump and/or in that the gas mixture employed originates from an EDC cracking process in which external EDC evaporation is used and an EDC elutriation stream is produced, the EDC elutriation stream being included in the quench bottom product treatment and
is either fed into the quench bottom product
or, in the first step of the quench bottom product treatment, the external heat supply is replaced by feeding-in with flash evaporation.